Cochlear lead

ABSTRACT

A cochlear lead includes a plurality of electrodes configured to stimulate an auditory nerve from within a cochlea and a flexible body supporting the plurality of electrodes along a length of the flexible body. A stiffening element is slidably encapsulated within the flexible body, the stiffening element extending past a most distal electrode at the tip of the cochlear lead, wherein a distal portion of the stiffening element plastically deforms upon insertion into a curved portion of the cochlea.

RELATED DOCUMENTS

The present application is a continuation of U.S. application Ser. No.13/463,450 (issued as U.S. Pat. No. 9,033,869) filed May 3, 2012entitled “Cochlear Lead” which is a continuation-in-part and claimsbenefit under 35 U.S.C. §120 of U.S. application Ser. No. 12/789,264,(issued as U.S. Pat. No. 9,037,267) filed May 27, 2010, entitled“Cochlear Lead” to Chuladatta Thenuwara et al. These applications areincorporated herein by reference in its entirety.

BACKGROUND

Hearing loss can be corrected using a number of approaches, includingsurgically placing a cochlear implant which includes an electrode arraythat is inserted into the cochlea of a patient. The electrode arraypresents electrical stimulation directly to auditory nerve fibers in thecochlea. This leads to the perception of sound in the brain and providesat least partial restoration of hearing function. Occasionally, thecochlear implant may need to be replaced with a new cochlear implant.The original electrode array is removed from the cochlea and a newcochlear lead is inserted. In some instances, the cochlea may havetissue that at least partially occludes the passageway into which thenew cochlear lead is to be inserted. This presents a number ofchallenges that can be addressed by a new cochlear lead design.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of theprinciples described herein and are a part of the specification. Theillustrated embodiments are merely examples and do not limit the scopeof the claims.

FIG. 1 is a diagram showing an illustrative cochlear implant system inuse, according to one example of principles described herein.

FIG. 2 is a diagram showing external components of an illustrativecochlear implant system, according to one example of principlesdescribed herein.

FIG. 3 is a diagram showing the internal components of an illustrativecochlear implant system, according to one example of principlesdescribed herein.

FIG. 4 is a cross-sectional view of a cochlea with an illustrativecochlear lead which includes an electrode array and stiffening element,according to one example of principles described herein.

FIG. 5A is a partial side view of an illustrative stiffening element andsheath, according to one example of principles described herein.

FIG. 5B shows a cross-section of an illustrative stiffening element witha sheath in place and a cap placed over the distal end of the stiffeningelement, according to one example of principles described herein.

FIG. 5C shows a cross-section of a tip of an illustrative cochlear lead,according to one example of principles described herein.

FIG. 6A is a side view of an illustrative cochlear lead, according toone example of principles described herein.

FIGS. 6B-6C are cross-sectional views of the cochlear lead shown in FIG.6A, according to one example of principles described herein.

FIGS. 7A-7C are cross sectional diagrams of steps in an illustrativeprocess for making a cochlear lead with an integral stiffening element,according to one example of principles described herein.

FIG. 8 is a flowchart showing an illustrative method for forming acochlear lead, according to one example of principles described herein.

FIG. 9 is a flowchart showing an illustrative method for using acochlear lead, according to one example of principles described herein.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

In human hearing, hair cells in the cochlea respond to sound waves andproduce corresponding auditory nerve impulses. These nerve impulses arethen conducted to the brain and perceived as sound.

Hearing loss, which may be due to many different causes, is generally oftwo types: conductive and sensorineural. Conductive hearing losstypically occurs where the normal mechanical pathways for sound to reachthe hair cells in the cochlea are impeded, for example, from damage tothe ossicles. Conductive hearing loss may often be helped by usingconventional hearing aids that amplify sounds so that acousticinformation can reach the cochlea and the hair cells. Some types ofconductive hearing loss are also treatable by surgical procedures.

Many people who are profoundly deaf, however, have sensorineural hearingloss. This type of hearing loss can arise from the absence or thedestruction of the hair cells in the cochlea which then no longertransduce acoustic signals into auditory nerve impulses. Individualswith sensorineural hearing loss may be unable to derive significantbenefit from conventional hearing aid systems alone, no matter how loudthe acoustic stimulus is. This is because the mechanism for transducingsound energy into auditory nerve impulses has been damaged. Thus, in theabsence of properly functioning hair cells, auditory nerve impulsescannot be generated directly from sounds.

To overcome sensorineural deafness, cochlear implant systems, orcochlear prostheses, have been developed that can bypass the hair cellslocated in the cochlea by presenting electrical stimulation directly tothe auditory nerve fibers. This leads to the perception of sound in thebrain and provides at least partial restoration of hearing function.Most of these cochlear prosthesis systems treat sensorineural deficit bystimulating the ganglion cells in the cochlea directly using animplanted lead that has an electrode array. Thus, a cochlear prosthesisoperates by directly stimulating the auditory nerve cells, bypassing thedefective cochlear hair cells that normally transduce acoustic energyinto electrical activity to the connected auditory nerve cells.

A cochlear implant system typically comprises both an external unit thatreceives and processes ambient sound waves and an implantedprocessor/cochlear lead that receives data from the external unit anduses that data to directly stimulate the auditory nerve. The cochlearlead includes an electrode array that is implanted within one of thecochlear ducts, such as the scala tympani. To minimize damage tosensitive tissues within the patient's cochlea, it can be desirable forthe electrode array to be accurately placed within the cochlea using aminimum amount of insertion force. The cochlear implant should bedesigned so that the insertion forces do not kink or otherwise damagethe delicate wires and electrodes contained within the implant.

According to one illustrative embodiment, the electrode array can beconstructed from biocompatible silicone, platinum-iridium wires, andplatinum electrodes. The portion of the lead to be inserted into thecochlea is designed to be relatively flexible so that it can curvearound the helical interior of the cochlea.

This specification describes a stiffening element that provides adesired level of rigidity to the electrode array for improved control bythe surgeon and prevents kinking along the length of the cochlear leadduring insertion. In some embodiments, the stiffening element extendspast the most distal electrode to the tip of the electrode array. Thisstiffening element can be particularly suited for revision surgeries.The stiffening element is more rigid than the body of the cochlear leadand can be fully encapsulated within the lead to reduce the risk ofinfection and better stabilize it within the lead. According to oneillustrative embodiment, this stiffening element serves at least fourpurposes. First, the stiffening element provides additional rigidityalong the length of the cochlear lead, thereby reducing the likelihoodthat the insertion forces will kink the lead. Second, the stiffeningelement provides the surgeon with greater control over the placement ofthe lead within the cochlea. Third, the stiffening element redirects theinsertion force into a tangential force, which allows the cochlear leadto be inserted deeper into the cochlea with less applied force. Fourth,at least a portion of the stiffening element may be formed from amaterial that plastically deforms during insertion, which allows thestiffening element to conform to the shape of the cochlea and providesthe ability to overcome obstructive tissue and provide full insertion ofthe electrode array.

Revision surgery can be used to explant a cochlear electrode array fromthe cochlea and replace it with a new electrode array. During revisionsurgery it is sometimes difficult to insert a compliant pre-curved orlateral electrode array due to the presence of scar tissue/ossificationthat has formed around the previous electrode array. For a partiallyossified cochlea or for a cochlea with fibrous tissue growth, a stifferelectrode can be used to ensure insertion to the full depth of theelectrode. In one example, a cochlear lead includes an integralstiffener with suitable malleability. The integral stiffener extendsalong the complete length of the electrode array and conforms to theshape of the cochlea.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present systems and methods. It will be apparent,however, to one skilled in the art that the present systems and methodsmay be practiced without these specific details. Reference in thespecification to “an embodiment,” “an example,” or similar languagemeans that a particular feature, structure, or characteristic describedin connection with the embodiment or example is included in at leastthat one embodiment, but not necessarily in other embodiments. Thevarious instances of the phrase “in one embodiment” or similar phrasesin various places in the specification are not necessarily all referringto the same embodiment.

FIG. 1 is a diagram showing one illustrative embodiment of a cochlearimplant system (100) having a cochlear implant (300) with an electrodearray (195) that is surgically placed within the patient's cochlea.Ordinarily, sound enters the external ear, or pinna, (110) and isdirected into the auditory canal (120) where the sound wave vibrates thetympanic membrane (130). The motion of the tympanic membrane isamplified and transmitted through the ossicular chain (140), whichconsists of three bones in the middle ear. The third bone of theossicular chain (140), the stirrup (145), contacts the outer surface ofthe cochlea (150) and causes movement of the fluid within the cochlea.Cochlear hair cells respond to the fluid-borne vibration in the cochlea(150) and trigger neural electrical signals that are conducted from thecochlea to the auditory cortex by the auditory nerve (160).

As indicated above, the cochlear implant (300) is a surgically implantedelectronic device that provides a sense of sound to a person who isprofoundly deaf or severely hard of hearing. In many cases, deafness iscaused by the absence or destruction of the hair cells in the cochlea,i.e., sensorineural hearing loss. In the absence of properly functioninghair cells, there is no way auditory nerve impulses can be directlygenerated from ambient sound. Thus, conventional hearing aids, whichamplify external sound waves, provide no benefit to persons sufferingfrom complete sensorineural hearing loss.

Unlike hearing aids, the cochlear implant (300) does not amplify sound,but works by directly stimulating any functioning auditory nerve cellsinside the cochlea (150) with electrical impulses representing theambient acoustic sound. Cochlear prosthesis typically involves theimplantation of electrodes into the cochlea. The cochlear implantoperates by direct electrical stimulation of the auditory nerve cells,bypassing the defective cochlear hair cells that normally transduceacoustic energy into electrical energy.

External components (200) of the cochlear implant system can include aBehind-The-Ear (BTE) unit (175), which contains the sound processor andhas a microphone (170), a cable (177), and a transmitter (180). Themicrophone (170) picks up sound from the environment and converts itinto electrical impulses. The sound processor within the BTE unit (175)selectively filters and manipulates the electrical impulses and sendsthe processed electrical signals through the cable (177) to thetransmitter (180). The transmitter (180) receives the processedelectrical signals from the processor and transmits them to theimplanted antenna (187) by electromagnetic transmission. In somecochlear implant systems, the transmitter (180) is held in place bymagnetic interaction with a magnet (189) in the underlying antenna(187).

The components of the cochlear implant (300) include an internalprocessor (185), an antenna (187), and a cochlear lead (190) having anelectrode array (195). The internal processor (185) and antenna (187)are secured beneath the user's skin, typically above and behind thepinna (110). The antenna (187) receives signals and power from thetransmitter (180). The internal processor (185) receives these signalsand performs one or more operations on the signals to generate modifiedsignals. These modified signals are then sent through the cochlear lead(190) to the electrode array (195), which is the portion of the cochlearlead (190) that is implanted within the cochlea (150) and provideselectrical stimulation to the auditory nerve (160).

The cochlear implant (300) stimulates different portions of the cochlea(150) according to the frequencies detected by the microphone (170),just as a normal functioning ear would experience stimulation atdifferent portions of the cochlea depending on the frequency of soundvibrating the liquid within the cochlea (150). This allows the brain tointerpret the frequency of the sound as if the hair cells of the basilarmembrane were functioning properly.

The cochlear lead typically comprises an electrode array that isimplanted in the scala tympani. The electrode array typically includesseveral separately connected stimulating electrode contacts,conventionally numbering about 6 to 30, longitudinally disposed on athin, elongated, flexible carrier. The electrode array is pushed intothe scala tympani duct in the cochlea, typically to a depth of about 13to 30 mm via a cochleostomy or via a surgical opening made in the roundwindow at the basal end of the duct.

In use, the cochlear electrode array delivers electrical current intothe fluids and tissues immediately surrounding the individual electrodecontacts to create transient potential gradients that, if sufficientlystrong, cause the nearby auditory nerve fibers to generate actionpotentials. The auditory nerve fibers branch from cell bodies located inthe spiral ganglion, which lies in the modiolus, adjacent to the insidewall of the scala tympani. The density of electrical current flowingthrough volume conductors such as tissues and fluids tends to be highestnear the electrode contact that is the source of such current.Consequently, stimulation at one contact site tends to selectivelyactivate those spiral ganglion cells and their auditory nerve fibersthat are closest to that contact site.

FIG. 2 is an illustrative diagram showing a more detailed view of theexternal components (200) of one embodiment of a cochlear implantsystem. External components (200) of the cochlear implant system includea BTE unit (175), which comprises a microphone (170), an ear hook (210),a sound processor (220), and a battery (230), which may be rechargeable.The microphone (170) picks up sound from the environment and converts itinto electrical impulses. The sound processor (220) selectively filtersand manipulates the electrical impulses and sends the processedelectrical signals through a cable (177) to the transmitter (180). Anumber of controls (240, 245) adjust the operation of the processor(220). These controls may include a volume switch (240) and programselection switch (245). The transmitter (180) receives the processedelectrical signals from the processor (220) and transmits theseelectrical signals and power from the battery (230) to the cochlearimplant by electromagnetic transmission.

FIG. 3 is an illustrative diagram showing one embodiment of a cochlearimplant (300), including an internal processor (185), an antenna (187),and a cochlear lead (190) having an electrode array (195). The cochlearimplant (300) is surgically implanted such that the electrode array(195) is internal to the cochlea, as shown in FIG. 1. The internalprocessor (185) and antenna (187) are secured beneath the user's skin,typically above and behind the pinna (110), with the cochlear lead (190)connecting the internal processor (185) to the electrode array (195)within the cochlea. According to one illustrative embodiment, theelectrode array (195) is straight or slightly curved before beinginserted into the cochlea (150). As discussed below, this particularelectrode array (195) is designed for revision surgery. However, theprinciples described can be applied to a broad range of medical devices.As discussed above, the antenna (187) receives signals from thetransmitter (180) and sends the signals to the internal processor (185).The internal processor (185) modifies the signals and passes themthrough the cochlear lead (190) to the electrode array (195). Theelectrode array (195) is inserted into the cochlea and provideselectrical stimulation to the auditory nerve. This provides the userwith sensory input that is a representation of external sound wavessensed by the microphone (170).

FIG. 4 is a partially cutaway perspective view of a cochlea (150) andshows an illustrative electrode array (195) being inserted into thecochlea (150). The primary structure of the cochlea is a hollow,helically coiled, tubular bone, similar to a nautilus shell. The coiledtube is divided through most of its length into three fluid-filledspaces (scalae). The scala vestibuli (410) is partitioned from the scalamedia (430) by Reissner's membrane (415) and lies superior to it. Thescala tympani (420) is partitioned from the scala media (430) by thebasilar membrane (425) and lies inferior to it. A typical human cochleaincludes approximately two and a half helical turns of its constituentchannels. The cochlear lead (190) is inserted into one of the scalae,typically the scala tympani (420), to bring the individual electrodesinto close proximity with the tonotopically organized nerves.

Throughout the specification and appended claims the term “distal”refers to portions that are closer to the tip (440) of the cochlear lead(190) and “proximal” refers to portions that are farther away from thetip (440). The terms “medial” and “lateral” refer to locations that arecloser to the center of the cochlea and closer to the outer portions ofthe cochlea, respectively. For example, the phrase medial wall of thecochlea refers to portions of a cochlear duct that are toward the centerof the cochlea.

In the example shown in FIG. 4, the scala tympani (420) is narrowed byossification (425). The ossification and other obstructions in the scalamay form for a variety of reasons, including foreign body reaction to aprevious electrode array. The ossification or other obstructions maypartially block the scala and make the insertion of a new electrodearray during revision surgery challenging. In particular, the surgeonmay need increased control to maneuver the electrode array around orthrough the obstructions. Higher forces may be needed to successfullyinsert the electrode array to the desired insertion depth. Thecombination of higher forces and obstructions can lead to kinking orfolding of the electrode array. In one embodiment, it can be desirableto insert the electrode array to 360 degrees or approximately the samedepth as the previous electrode array.

The illustrative cochlear lead (190) is specifically designed to providethe surgeon with the desired control and prevent the electrode arrayfrom kinking or folding along its length. The cochlear lead (190)includes a lead body (445). The lead body (445) connects the electrodearray (195) to the internal processor (185, FIG. 3). A number of wires(455) pass through the lead body (445) to bring electrical signals fromthe internal processor (185, FIG. 3) to the electrode array (195).According to one illustrative embodiment, proximal of the electrodearray (195) is a molded silicone rubber feature (450). The feature (450)can serve a variety of functions, including, but not limited to,providing a structure that can be gripped or pushed by an insertion tooland providing a visual indicator of how far the cochlear lead (190) hasbeen inserted.

The wires (455) that conduct the electrical signals generated by theprocessor are connected to the electrodes (465) within the electrodearray (195). For example, electrical signals which correspond to a lowfrequency sound may be communicated via a first wire to an electrodenear the tip (440) of the electrode array (195). Electrical signalswhich correspond to a high frequency sound may be communicated by asecond wire to an electrode (465) near the proximal end of the electrodearray (195). According to one illustrative embodiment, there may be onewire (455) for each electrode (465) within the electrode array (195).The internal processor (185, FIG. 3) may then control the electricalfield generated by each electrode individually. For example, oneelectrode may be designated as a ground electrode. The remainder of theelectrodes may then generate electrical fields which correspond tovarious frequencies of sound. Additionally or alternatively, adjacentelectrodes may be paired, with one electrode serving as a ground and theother electrode being actively driven to produce the desired electricalfield.

According to one illustrative embodiment, the wires (455) and portionsof the electrodes (465) are encased in a flexible body (475). Theflexible body (475) may be formed from a variety of biocompatiblematerials, including, but not limited to, medical grade silicone rubber.The flexible body (475) secures and protects the wires (455) andelectrodes (465). The flexible body (475) allows the electrode array(195) to bend and conform to the geometry of the cochlea. When placedwithin the cochlea (150), the electrode array (195) brings theindividual electrodes into close proximity with the tonotopicallyorganized nerves in the cochlea (150).

Additionally, as can be seen in FIG. 4, and described further below, astiffening element (500) extends from the molded silicone rubber feature(450) to the tip (440) of the electrode array (195). The stiffeningelement (500) allows the cochlear lead to be more precisely positionedwithin the cochlea and reduces the propensity of the cochlear lead (190)to kink. In embodiments where the stiffening element (500) isplastically deformable, the stiffening element (500) conforms to thecurvature of the cochlea during insertion. This can help preventundesirable motion of the lead within the cochlea. The stiffeningelement and its function are described in more detail below.

FIGS. 5A and 5B are detail views of an illustrative stiffening element.FIG. 5A is a side view of an illustrative stiffener (515) and sheath(540). The sheath (540) is placed over the stiffener (515) to form thestiffening element. According to one illustrative embodiment, the sheath(540) may be formed from PTFE or other polymer. For example, the sheath(540) may be formed from expanded PTFE. Expanded PTFE has high strength,chemical inertness, and a much lower modulus of elasticity thanunexpanded PTFE. According to one illustrative embodiment, the expandedPTFE sheath or liner may be a tube with an inside diameter of 0.006inches and an outside diameter of 0.007 inches. Additionally, the sizeand shape of the sheath (540) may be selected to allow more or lessrelative motion between the stiffening element (500) and the sheath andbetween the sheath and the flexible body (475). For example, in someembodiments, the sheath (540) may be sized to fit relatively looselyover the stiffening element (500). This will allow the stiffeningelement (500) to slide within the sheath (540) relatively easily.Alternatively, the sheath (540) may be sized to fit more tightly aroundthe stiffening element (500). In this embodiment, there may be little orno relative motion of the stiffening element (500) within the sheath(540). Instead, the relative motion of the stiffening element (500) withrespect to the flexible body (475, FIG. 5) may take place primarily atthe interface between the outer surface of the sheath (540) and theinner surface of the flexible body.

In some embodiments, the fit of the sheath (540) over the stiffeningelement may vary from location to location. For example, the sheath(540) may be formed so that it fits relatively loosely over thestiffening element (500) near its proximal and distal ends, but have atighter fit in the center of the stiffening element. Additionally, thesheath (540) may cover the entire stiffening element (500) or only aportion of the stiffening element (500). For example, the sheath (540)may cover only the distal portion (515) of the stiffening element.Although the sheath (540) is shown with an open end and a closed end,the sheath may have both ends open or both ends closed.

FIG. 5B shows a cross-section of an illustrative stiffening element(500) with the sheath (540) in place over the stiffener (515). A cap(545) has been place over the distal end of the stiffening element. Thecap (545) may be formed from the same or different material than thesheath (540). The cap (545) may be bonded to the tip of the stiffeningelement (500) or may be an integral part of the sheath (540).

FIG. 5C shows a cross-section of a distal portion of an illustrativecochlear lead. As shown in the figure, the stiffening element extendspast the most distal electrode (465-1), with the cap (540) being locatedbetween the most distal electrode (465-1) and the tip (440) of theelectrode array. The cap (545) may serve a number of purposes, includingdistributing the insertion forces over a larger area and preventing thestiffening element (500) from penetrating the flexible body (475).

FIG. 6A is a partial side view of an illustrative cochlear lead (190).As discussed above, the cochlear lead (190) includes an electrode array(195) comprising electrodes (465), a lead body (445) carrying wires(455) that extend from the internal processor (185, FIG. 3) to theelectrodes (465), a flexible body (475) on which electrodes (465) aredisposed, a stiffening element (500) disposed within a portion of theflexible body (475), and a molded silicone rubber feature (450) proximalof the electrode array (195). The illustrative cochlear lead (190)further includes a cochleostomy marker (466) as a guide for positioningthe electrode array (195). When the electrode array (195) is properlypositioned within the cochlea, the cochleostomy marker (466) ispositioned at or near the cochleostomy, and, the electrodes (465) arewell positioned to stimulate the tonotopically-arranged groups of nerveendings.

The stiffening element (500) may be located within the flexible body(475) and extend from at or near the molded feature (450) to a locationwithin the electrode array (195). According to one embodiment, thestiffening element (500) comprises or consists essentially of platinum.In other embodiments, it may be made of any material which provides thedesired mechanical and chemical properties. These properties may includelow yield strength and chemical inertness. By way of example and notlimitation, the stiffening element (500) may be a plastic, metal, glass,composite, or other material. According to one illustrative embodiment,the stiffening element (500) may comprise or consist of gold or a goldalloy.

According to one embodiment, stiffening element (500) extendsapproximately 2 mm to 6 mm from the cochleostomy. For example, thestiffening element may extend into the cochlea approximately 15 mm to 30mm from the cochleostomy. In this illustrative embodiment, thestiffening element (500) extends from the molded rubber gripping feature(450) through the electrode array past the most distal electrode. Thegripping feature (450) can be grasped by general or specialized surgicaltools. The extension of the stiffening element into the gripping feature(450) allows these tools to grip the stiffening element through theencapsulation with minimal risk of damage to other components in thecochlear lead. The stiffening element can then be manipulated using thesurgical tools to guide the cochlear lead into the desired positionwithin the cochlea.

FIG. 6B shows a cross-section along line 6B-6B of a portion of theillustrative cochlear lead (190) in which the stiffening element (500)is disposed. The wires (455) may be shaped into a wire bundle by theelectrode (465). Portions of the electrode, the wires, and thestiffening element (500) are encapsulated by the flexible body (475). Inthis particular embodiment, the electrodes (465) are disposed within theflexible body (475) on the medial wall of the electrode array. Thestiffening element (500) is disposed in flexible body (475) opposite theelectrodes (465).

FIG. 6C is a cross-sectional diagram along line 6C-6C taken through thedistal end of the stiffening element (500). Because the distal end ofthe stiffening element (500) extends beyond the most distal electrode(465-1), only the cross section of the stiffening element (500) is shownwithin the flexible body (475). The cross sectional view shows thestiffener (515) surrounded by the sheath (540) and the cap (545).

The stiffness and ductility of the stiffening element (500) can beselected to significantly influence the amount of force required toinsert the electrode array (195) into the cochlea. Alternatively oradditionally, the geometry of the stiffening element can be alteredalong its length to create the desired mechanical properties. Forexample, the distal portion of the stiffener may have a variety of crosssectional geometries, including flattened, elliptical or circular. Thecross sections may vary along the length of the stiffener. These crosssections can be selected to produce the desired stiffness, with lowerstiffnesses typically desired near the distal end of the stiffener. Thedifferent cross sections may be formed in a variety of ways, includinggrinding, rolling, pressing, drawing, or other suitable technique. Insome embodiments, the distal portion of the stiffener may have a numberof micro-machined features that produce desired bending characteristics.

FIGS. 7A-7C are cross sectional diagrams of steps in an illustrativeprocess for making a cochlear lead with an integral stiffening element.In a first step, a stiffener (700) is shaped and annealed. The shapingprocess may include cutting the stiffener (700) to the desired length.In embodiments where the stiffening element is a metal, the distalportion (515) may be annealed. Annealing is a versatile heat treatingprocess which alters the material properties (such as yield strength andductility) of a metal. According to one illustrative example, thestiffener (700) may include platinum or a platinum alloy. In someembodiments, the stiffener (700) may consist essentially of platinum.Annealing may be used to produce a distal portion (710) having greaterductility and lower stiffness than the rest of the stiffener (700). Thisallows the distal portion to bend more easily to follow the curvature ofthe cochlear ducts. Further, annealing will allow the distal portion tomaintain its bent shape after insertion into the cochlea. This canassist in holding the electrode array in place within the cochlea. Forexample, a distal portion (515) with high yield strength of 185 to 205MPa may be more susceptible to migrate out from the cochlea due tostrain energy. Conversely, a distal portion with a lower yield strengthof 14 to 35 MPa may plastically conform to the shape of the cochlea andhave a tendency to retain the electrode array in the cochlea.

A variety of other techniques, such as work hardening, could be used tomodify the yield strength, malleability, ductility, stiffness, or othercharacteristics of the stiffener (700). According to one example, thedistal portion (710) of the stiffener (700) is annealed and the tipportion (705) is annealed so that the metal is “dead soft.” The term“dead soft” refers to the condition of maximum softness that isattainable in a metal or metal alloy through annealing. In otherembodiments, the distal portion may be annealed but not to the extentthat the metal is dead soft.

The annealing may be performed in a stepwise fashion, the entire tipportion (705) having a substantially uniform dead soft anneal and theremainder of the distal portion (710) being annealed to a lesser extent.The temper of the remainder of the stiffener (700) may remain in an “asdrawn” state or may be heat treated to alter characteristics of themetal. In one embodiment, portions of the electrode that will be bent tofollow the spiral of the cochlea are annealed and portions that remainin the relatively straight basal portion of the cochlea are more rigid.In alternative embodiments, the annealing may vary continuously alongthe length of the stiffener (700). According to one embodiment, thestiffener may be annealed from the distal tip up to approximately itsmidpoint.

The sheath (715) is formed by cutting it to length. In this example, thedistal end (725) is closed while the proximal end (720) of the sheath isopen. The sheath (715) is cut slightly longer than the stiffener. Asdiscussed above, the sheath (715) may be sized to allow for motion ofthe stiffener (700) within it.

FIG. 7B shows the stiffener (700) inserted into the sheath (715). Theproximal end (720) of the sheath is then fused or adhered shut. Thestiffener is then entirely enclosed in the sheath to form the stiffeningelement (500). In some embodiments, the sheath may be substantiallyimpermeable.

FIG. 7C is a side view of an illustrative cochlear lead (190) thatincludes the stiffening element (500), with the proximal end of thestiffening element (500) extending into the molded silicone rubberfeature (450) and its distal end (725) extending to the tip (440) of thecochlear lead (190).

The sheath is only one illustrative example. In other examples, thestiffener (515) may be overcoated with a thin layer. This thin layer maybe formed from a number of materials and applied in a variety of ways.In other embodiments, the flexible body may include a lumen may be linedwith polytetrafluoroethylene (PTFE) or other friction reducing material.The stiffener can then be inserted into the lumen. In most cases, thestiffener can move relative to the flexible body (475, FIG. 4) duringbending. This reduces the overall bending stiffness of the cochlear lead(190) while still maintaining its resistance to kinking.

FIG. 8 is a flowchart showing an illustrative method for forming acochlear lead. A stiffener is formed with the desired material,diameter, and length. For example, the stiffener may comprise orsubstantially consist of platinum. The distal portion of the stiffeneris then annealed (step 805) so that the distal portion of the stiffeneris substantially softer than a proximal portion. In some embodiments,the distal portion is approximately half of the total length thestiffener. In some implementations a tip portion of the stiffener may beannealed so that it is dead soft. The tip portion may be one quarter orless of the total length of the stiffener.

The sheath is then formed. As discussed above, the sheath may be apolymer tube that is selected to allow the stiffener to slide freelywithin its inner diameter. Additionally, the polymer material may beselected for its lubricity and biocompatibility. One end of the tube isclosed, with the other end remaining open to receive the stiffener.

In some embodiments an additional cap is placed over the distal end ofthe sheath. The cap may be a flat plate or semi spherical shape thatprevents puncture of the stiffening element through the flexible body.The cap may be used alone or in conjunction with the sheath. In otherembodiment, the sheath is fused together to form a ball or caplikestructure.

The stiffener is inserted into the sheath (step 810) and the ends of thesheath are closed (step 815). The sheath and the stiffener make up thestiffening element. The stiffening element is then encapsulated in theflexible body (step 820) with the electrodes and wires. A variety oftechniques can be used to encapsulate the stiffening element in theflexible body. Illustrative examples of these techniques are describedin U.S. application Ser. No. 12/789,264, filed May 27, 2010, entitled“Cochlear Lead” to Chuladatta Thenuwara et al., which was incorporatedby reference above.

A variety of other steps can be taken to complete the manufacture of thecochlear lead. For example, these steps may include testing,sterilization, and packaging.

FIG. 9 is a flowchart showing an illustrative method for surgicallyimplanting using a cochlear lead with a full length stiffening elementduring a revision surgery. The patient is prepared and the appropriatesurgical openings made. The existing cochlear implant is removed fromthe patient, including removing a previous cochlear lead from thecochlea (step 905). The new cochlear implant, including a replacementcochlear lead with a full length slidably encapsulated stiffeningelement, is taken from its packaging (step 910). The replacementcochlear lead includes an offset gripping feature. The stiffeningelement extends from within the offset gripping feature through theelectrode array and past the most distal electrode. In some instances,the new cochlear electrode array may have a smaller diameter than thecochlear electrode which was removed from the patient. This may allowfor the new cochlear electrode to be more easily inserted into theopening that previously contained the old cochlear electrode.

The surgeon attaches the appropriate tool to the cochlear lead bygripping a proximal end of the stiffening element contained within thegripping feature (step 915). This tool may be a special purpose toolthat is specifically adapted for insertion of the cochlear lead or maybe a more general purpose surgical tool such as tweezers or forceps. Insome cases, the surgeon may manually bend the distal portion of thecochlear lead to achieve the desired curvature. Because the distalportion of the cochlear lead is annealed, the cochlear lead will tend tomaintain the curvature created by the surgeon. In other examples, thesurgeon may leave the cochlear lead in its substantially straightconfiguration.

The surgeon then inserts the electrode array into the patient's cochleaby manipulating the proximal end of the stiffening element to guide thecochlear lead into the cavity vacated by the previous cochlear lead(step 920). The full length stiffening element provides the surgeon withincreased control throughout the insertion process and allows slightlyhigher forces to be used without kinking or folding over the electrodearray. The surgeon can use the increased control and resistance tokinking provided by the stiffening element to maneuver the cochlear leadpast obstructions to a predetermined depth in the cochlea.

The preceding description has been presented only to illustrate anddescribe embodiments and examples of the principles described. Thisdescription is not intended to be exhaustive or to limit theseprinciples to any precise form disclosed. Many modifications andvariations are possible in light of the above teaching.

What is claimed is:
 1. An implantable lead comprising: a plurality ofelectrodes; a flexible body supporting the plurality of electrodes alonga length of the flexible body; a lumen in the flexible body configuredto receive a stiffening element, the lumen extending a majority of thelength of the flexible body; a stiffening element; and a cap for adistal tip of the stiffening element, wherein the cap has a diametergreater than a diameter of the stiffening element.
 2. The lead of claim1, wherein the stiffening element further comprises a sheath covering adistal portion of the stiffening element.
 3. The lead of claim 2,wherein the cap is an integral part of the sheath.
 4. The lead of claim3, wherein the cap is fused from a portion of the sheath.
 5. The lead ofclaim 2, wherein the cap is attached to a sheath with adhesive.
 6. Thelead of claim 2, wherein the cap is thicker than a wall of the sheath.7. The lead of claim 2, wherein the cap comprises a different materialthan the sheath.
 8. The lead of claim 1, wherein the cap is flat.
 9. Thelead of claim 1, wherein the cap is cup shaped.
 10. The lead of claim 1,wherein the cap has a semi spherical shape.
 11. The lead of claim 1,wherein the cap has a stiffness less than a stiffness of the stiffeningelement.
 12. The lead of claim 1, wherein the cap has a stiffnessgreater than a stiffness of the flexible body.
 13. The lead of claim 1,wherein a cross-sectional area of the cap is greater than across-sectional area of the stiffening element such that the cap reducespressure applied by the stiffening element to the flexible body at adistal end of the lumen.
 14. The lead of claim 1, wherein the cap isconfigured to receive a distal end of a stiffening element.
 15. The leadof claim 1, wherein the cap is bonded to an end of a stiffening element.16. The lead of claim 1, wherein the cap moves with the stiffeningelement.
 17. An electrode array for nerve stimulation comprising: anelongate flexible body; a plurality of electrodes spaced along theflexible body; a lumen, open at a proximal end of the flexible body andrunning within the flexible body for a majority of the length of theflexible body; and a cap located at a distal end of a stylet so as toimpede the stylet from advancing past a distal end of the lumen, whereinthe cap has a diameter greater than a diameter of the stylet.
 18. Theelectrode array of claim 17, wherein the cap is composed of a firstmaterial and the flexible body is composed of a second material, thefirst and second materials being different.
 19. A lead for stimulation,the lead comprising: a flexible body with a distal end and a proximalend; a lumen in the flexible body, wherein the lumen extends through amajority of a length of the flexible body; at least one electrodelocated along a length of the flexible body; and a cap located between adistal end of the lumen and a distal end of the flexible body, the capcomprising a material with a greater stiffness than the flexible body,the cap placed to distribute a force from a stiffening element insertedinto the lumen and the cap having a diameter greater than a diameter ofthe stiffening element.
 20. The lead of claim 19, wherein the cap is cupshaped.